Application analysis of chrome plated steel plate in food packaging

The chrome-plated steel sheet, which is called chrome plate for short, is a new-type can-making material developed because of the low tin metal resources and high cost. The United States, Europe and Japan have more and more applications to replace some of the tin plate materials. There are relatively few applications in our country. This requires a correct understanding of this material and some technical and ideological analysis.

First, the development and characteristics of chrome plate

Chromium is a hard, silvery white shiny metal with good hardness, wear resistance, and matrix binding properties, as well as high heat resistance. As early as the 20th century, the chrome plating process was used for industrial production. In the United States, in 1940, chromium plating plates were researched, followed by chromic acid for chemical treatment and electrolytic chromic acid treatment, and transferred to mass production. After living in Japan from 1955 onwards, the chromate passivation method was studied and many results were obtained. For example, the Hitop chrome plate of Dongyang Steel has entered industrial production in 1961; Fuji Steel has also produced Con Cupper chrome plates in succession; in 1972, Japanese steel pipes produced Bright Coat chrome plates. board. European countries also successively produced chrome plates in the 1970s.

The original plate used for the chrome plate is the same as the tin plate. They are all low-carbon steel plates, but the chrome plating layer is very thin (< 1.3 μm). The process is the same as that of the tin plate. Therefore, in general, the original tin plating production line needs only a slight modification, and the addition of a chrome plating bath can achieve tin or chrome plating.

The chrome plated structure has four layers: a steel substrate, a metallic chromium layer, a hydrated chromium oxide layer, and an oil film. Since the substrates are the same, the mechanical properties are the same as those of tinplates, but there are some unique features in their application:

1. The cost is about 10% lower than that of tin plate. The appearance of gloss is not as good as that of tin plate.

2. Corrosion resistance is lower than that of tin plate. The coating is thin and the pin hole rate is high. Therefore, the inner and outer surfaces must be coated when using.

3. Chrome plate has strong adhesion and adhesion to organic coating is 3-6 times stronger than tin plate. Sulfidation corrosion resistance is also stronger than tinplate.

4. The chrome plate cannot be soldered and can only be lap welded or bonded.

5. The coating is thin and its toughness is poor. The cans are easy to be broken, so it is not suitable for redialing cans. It can be used for deep-flushing cans.

6. The chrome plate has high temperature resistance, generally no significant change in color and hardness at 5000 °C, and it begins to soften at 7000 °C.

Second, the main issues of application of chrome plate in food can making

From the above understanding, there are mainly two technical problems that restrict the application of chrome plating in the food can making industry:

One is the problem of corrosion resistance. This problem can be studied from the improvement of the chrome plating technology and the improvement of the post-plating paint. It is important to note that any technical improvements must take into account the cost to maintain the advantages of the chrome plate. For the moment, the corrosion resistance of the chrome plate is not as good as the tin plate, but it also has a certain corrosion resistance. It is suitable for the packaging of weak acidic foods. This kind of food market is very large, such as meat, fish, poultry, dairy products, vegetables, carrots, wrist beans, beets, potatoes and various beverages, soups and so on. If the coating is strictly technically guaranteed, some acidic foods with a PH of 3.7 to 4.5 can also be packaged in chrome plates, such as fruits, kimchi, and the like. It is worth noting that the corrosion resistance of chrome plating should not be limited to the general concept that chromium has good corrosion protection for iron. The chrome plating is a cathode coating and cannot play an electrochemical protective role. The porosity is high. Under the circumstances, the alloy effect between the steel substrate and the chromium layer is worth studying. Someone tested the corrosion resistance of the chromium steel in seawater and found that in the short-term (within 2 years) soaking, chromium has an effect on the corrosion resistance of the carbon steel; but long-term (Over 2 years later) Soaking is not only unhelpful but also accelerates corrosion.

The second is the can manufacturing process. The chrome plated coating is thin and has poor toughness, lacking the flushing of the lubricant like tin, so the flushing process needs careful study. At present, it is feasible to use deep-drawing process for two-piece cans. Since it is impossible to solder, three-piece cans must be welded or bonded. If a tinplate chrome plate is used in combination, a tin plate is used for the can body and a chrome plate is used for the bottom lid of the can. In addition, the sealing of the can body and the lid should not be double rolled into the process.

Third, the chrome plate application thinking and understanding

The above analysis shows that although the application of chrome plated in food packaging has its limitations, it also has its own characteristics and advantages, and it has a relatively broad market, so it can be worthy of promotion and development. However, as far as China's current reality is concerned, there are still two issues that are in the development process.

One of China's chrome plate manufacturing industry is almost blank. Since the reform and opening up, China's iron and steel industry has developed rapidly. At the end of the last century, China has become the world's top four steel producer. In 2002, the steel output reached 127 million tons. However, according to its variety and quality, it is quite different from the developed countries in the world. About 40% of the products can meet the needs of varieties and quality, 30% of the varieties are not enough, 20% of the varieties are not good, and 10% cannot be produced. In fact, China's steel products are mainly based on wire rods and profiles. Plates and pipes are relatively scarce. However, 25 percent of the steel manufacturers are small enterprises, and 33 percent of the steel production is small enterprises. They are outdated equipment, the process is backward, and the raw material composition is unstable. Large enterprises also face the period of institutional reform and technological renovation. Therefore, as a whole, the gap in the quality of steel products in China is still large, the chemical composition fluctuates greatly, the performance is unstable, the dimensional accuracy of finished products is not high, and there are many surface defects. Obviously, the use of users causes a high scrap rate. Many companies have imported advanced equipment from abroad, but they still need to import materials because domestic raw materials are not applicable. From the perspective of sheet steel materials, although it has grown rapidly every year, domestically produced 19,003.81 million tons in 2002, an increase of 10.20% over the same period of last year, but the performance consumption reached 39.23 million tons. Therefore, the thin steel sheet has a very large gap in China. It is conceivable that in this context, the packaging industry wants to use chrome plated steel and can only rely on imports. It is worth mentioning that at present, China's tin plate production capacity is surplus, unfortunately most of them are low block production lines, and products can only be used for dry food packaging. High-grade tinplates that can meet the canning and beverage packaging still rely on imports. It can be seen from this that the raw materials themselves restrict the development and progress of food packaging. This involves many scientific management and coordinated development issues in our progress.

Second, how can our food canners know their packaging containers? At present, food canning enterprises mainly use tinplates, and their food products, cans sales, and corresponding equipment and mechanisms are all based on canned tinplates. Any technological innovations and innovations involve the issue of corporate efficiency. It is related to the credibility of the market. Therefore, the application of chrome plate materials is a risk to them. There must be scientific and economic analysis, reliable conditional guarantees, and entrepreneurs with a spirit of reform. Foreign countries have ready experience and equipment, and whether they can adapt to the current situation in China must be carefully explored. The author has learned about individual plants. Due to the limitations of the enterprise mechanism, the factory manager dared not take the risk, because this is a major transformation. It can thus be seen that there are conditional issues in the acceptance of new things by Chinese enterprises, and there are also ideological and cognitive issues. The key is to establish a mechanism for continuous exploration and continuous improvement by enterprises. In particular, after China's entry into the WTO, it faces the internationalization of the market. This must be strengthened and it is urgently needed.